P. M{\o}ller (STScI, Baltimore. On assignment from the Space Science Department of ESA)

Of the various types of heavy element absorption lines that
appear in quasar spectra, considerable controversy has
surrounded the interpretation of the so-called {$z_{\rm abs}
\approx z_{\rm em}$} systems, defined as C{\small IV} doublets at redshifts
within $\Delta v=\pm5000$~km~s$^{-1}$\ of the emission redshift. As
reviewed most recently by Foltz, Chaffee, Weymann, and
Anderson, 1988, in QSO Absorption Lines: Probing the
Universe
Cambridge University Press, Cambridge, p.53, it is not
presently clear whether this class of absorption system is
external or intrinsic in nature. One possibility is that
such systems arise in the halos of galaxies located in a
cluster that includes the host galaxy of the quasar
(Weymann, Williams, Peterson, and
Turnshek, 1979, ApJ, 234
, 33). Alternatively, such
systems could be related to the so-called broad absorption-line
systems, and caused by absorption in matter ejected by
the quasar itself (Turnshek, 1988, in QSO Absorption Lines:
Probing the Universe
Cambridge University Press, Cambridge, p.17.

Recent studies of {$z_{\rm abs}
\approx z_{\rm em}$} systems
(Savaglio, D'Odorico, and M{\o}ller, 1994, A\&A, 281
, 331;
Petitjean, Rauch, and Carswell, 1994, A\&A, 291
, 29;
M{\o}ller, Jakobsen, and Perryman, 1994, A\&A, 287
, 719)
have revealed that the {$z_{\rm abs}
\approx z_{\rm em}$} systems are in fact
distinctly different from corresponding intervening systems, in that
they generally have extremely high metallicities. This suggests that
those systems are more likely to be related to BAL systems, rather
than to intervening absorbers. I shall briefly review the evidence for
the metallicities of those systems, as well as the
inverse luminosity correlation (M{\o}ller, Jakobsen, 1987, ApJ-Lett
320
, L75). Both of these results may provide important clues
to the distribution, nature, and history of the absorbing gas in the
AGN environment.